Noam Wasserman's "Founder Frustrations" blog has some excellent content that is based on research covering many companies. This means that the content often is more objectively "valid" than what you get from a blog that simply represents a single person's insights and experiences.
I also really enjoyed Marc Andreesen's old blog which had some excellent entries on startups, funding structure, etc - but seems to have disappeared off the net so I can't link to it any more...
Tricky question, because it will obviously depend on your field. For me (web development, but those blogs actually are broader than that):
In order of importance:
Joel on software : The king, such an inspiration.
Coding Horror : Very frequent posts, and great tweeter account.
Signal vs Noise (37 signals) : huge audience, always throwing points to make you think.
I cannot be without most of them. Dharmesh's OnStartup 's is a great one, but now I am hooked with this answer's site, even if it's not a blog. I guess as time evolves the quality of a blog changes too. That is why I found the need to start an aggregator selecting top content in one place at Paper.
I'll add to the excellent list above:
Obviously I'll second Onstartups, blog and this site, Joel's blog and their forum, and Paul Graham's essays.